1. Field of the Invention
The present invention relates to a method of making a printed wiring board and in more particular to a method of making a printed wiring board which makes use of a solder resist process.
2. Description of the Prior Art
A solder resist has been used for preventing the solder bridging of a printed wiring board. Moreover, since in making a high density printed wiring board used for mounting, thereon, surface mount devices in fine pitches, the use of a method exhibiting highly precise positioning and accurate image-forming ability is required, photolithography using a light-sensitive liquid solder resist has been used for making such a high density printed wiring board instead of the screen printing method.
A method for making a printed wiring board through the conventional photolithographic method will be explained below with reference to FIG. 1 which is a perspective view of a conventional printed wiring board produced in accordance with such a photolithographic method.
As has been shown in FIG. 1, a solder resist layer 1 is formed on a high density printed wiring board used for mounting, thereon, surface mount devices in fine pitches of 0.5 mm. The solder resist 1 is formed so as to cover circuits 2 for junction formed on the wiring board and to separate the circuits 2 from pads 3 for surface-mounting likewise formed on the board, separate the circuit 2 from the adjacent circuit 2 and separate the pad 3 from the adjacent pad 3 for the purpose of preventing the bridging which may occur during soldering.
FIGS. 2A to 2E are cross sectional views showing, in order, processes for explaining the making of a printed wiring board according to the conventional photolithographic method.
First, circuits 2 for junction and pads 3 for surface-mounting are formed on a substrate to give a printed wiring board 4 provided with circuit as shown in FIG. 2A.
Then a light-sensitive liquid solder resist is applied on the whole surface of the circuit-carrying printed wiring board 4 by the curtain flow coating method and dried in a circulating hot air oven maintained at 70.degree. to 100.degree. C. for 20 to 40 minutes to give a film 6a of the light-sensitive liquid solder resist as shown in FIG. 2B.
Thereafter, a mask film 7 carrying a desired pattern is put on the top of the solder resist film 6a while correctly positioning it with respect to the circuit-carrying printed wiring board 4 and exposed to ultraviolet rays 8 of 200 to 1000 mj/cm.sup.2 to selectively cure the solder resist film 6a as shown in FIG. 2C. At this stage, the light-sensitive liquid solder resist film 6a is selectively cured through the irradiation with the ultraviolet rays 8 to give a cured light-sensitive liquid solder resist film 6b, while the un-exposed area 6c of the light-sensitive liquid solder resist film 6a remains uncured (see FIG. 2D).
The uncured light-sensitive liquid solder resist film 6c is removed by dissolving with a developer such as a 1% sodium carbonate solution so as to leave only the cured light-sensitive liquid solder resist film 6b as shown in FIG. 2E and to thus give a conventional printed wiring board as shown in FIG. 1.
This conventional method suffers from the following problems upon forming a solder resist layer between pads arranged in fine pitches for surface-mounting.
(1) The accuracy of the positioning of a mask film relative to a circuit-carrying printed wiring board is at highest.+-.75 .mu.m. Therefore, if a solder resist layer is formed between pads for surface-mounting arranged in very fine pitches of the order of not more than 0.5 mm, the solder resist suffers discrepancy of position relative to the circuit-carrying printed wiring board and in the worst case, the solder resist film covers even a part of the surface of the pads for surface-mounting. This accordingly leads to substantial reduction of reliability of the soldering between surface mount device and the printed wiring board. PA1 (2) Since the width of the solder resist present between the pads for surface-mounting is very small, sufficient adhesion is not ensured between the solder resist and the circuit-carrying printed wiring board and this often leads to peeling off or delaminating of the solder resist film. As a result, a solder bridging is possibly formed between the adjacent pad for surface-mounting and a surface-mounted device.